The present invention relates to the general field of fuel metering units with variable geometrical aperture. It finds a preferential application to fuel metering units fitting out fuel supply circuits of an aircraft engine, and more particularly to circuits delivering fuel for feeding combustion injectors of the engine.
Generally, a fuel supply circuit of an aircraft engine comprises a pumping system consisting of a low pressure pump connected upstream to the fuel tank of the aircraft and associated downstream with a high pressure pump that is driven by the engine via an accessory gearbox (or AGB). Downstream from the high pressure pump, the fuel supply circuit is separated into two branches, one of the branches comprising a bypass control valve and a fuel metering unit allowing regulation of the fuel flow rate feeding the combustion injectors of the engine, and the other branch comprising a heat exchanger and sending the fuel towards variable-geometry actuators of the engine (for example, air discharge valves or control valves for vanes of variable pitch). The excess fuel flow rate generated by the high pressure pump is returned to the inlet of this pump through the bypass control valve, which partially contributes to heating up the circuit, particularly at low speeds of the engine where the recycling rate is maximal.
Such a fuel supply circuit needs to be protected against the risk of icing of the water contained in the fuel. Indeed, the pressure and the temperature encountered during flight may cause water to pass from the liquid state to the solid state and cause malfunctions of the fuel supply circuit.
In conventional architectures of fuel supply circuits, the units that are the most sensitive to icing are supplied with heated fuel at a positive temperature by a heat exchanger. However, when oil is used for heating the fuel, the heat power available in the oil system of the engine makes it possible with a dedicated oil/fuel exchanger to heat the fuel sufficiently only for the low flow rates sent to the variable-geometry actuators of the engine. Indeed, this heat power is not sufficient for heating the fuel at high flow rates through the fuel metering unit, for example during stages when the aircraft is taking off or climbing.
Unfortunately, the units constituted by the bypass control valve, the fuel metering unit, and the combustion injectors of the engine may be sensitive to icing of the fuel.
In order to make the bypass control valve less sensitive to icing of the fuel, it is known to have resort to a servo-controlled valve with a large force margin and with a heated pressure difference detector at the terminals of the valve. Combustion injectors can generally operate for a significant length of time under icing conditions before leading to malfunction of the engine. This time is generally longer than the duration of the portion of the mission in which fuel is to be found under icing conditions.
In contrast, the fuel metering unit is a unit that it is difficult to make insensitive to icing of the fuel, in particular when the fuel metering unit is provided with a metering slot having a flow passage of profile that is exponential. Such a fuel metering unit has the advantage of making it possible to obtain high accuracy in the metering of fuel at low flow rates. However, when it is supplied with fuel under icing conditions, the narrow portion of the metering slot tends to become partially clogged because of particles of ice building up therein. More specifically, this partial clogging typically appears for high flow rates in the narrow portion of the slot (when the fuel is at a negative temperature). The result of this is that when returning to low flow rates after operating with a high flow rate (typically when returning to an idling stage of the engine), there exists a non-negligible risk that the injected fuel flow rate becomes less than the limit set by the burn-out limit, and therefore there is a risk of flame-out in the combustion chamber of the engine.
Published patent application FR 2 825 120 A1 discloses a fuel metering unit provided with a metering slot having a flow passage of profile that is exponential, the metering unit being of cylindrical shape with a movable element in the form of a piston. The metering slot is split into two portions that are circumferentially spaced apart from each other and that are separated by sealing means, so as to produce an outlet for low flow rates that is separated from the outlet for high flow rates. The outlet dedicated to low flow rates corresponds to the narrow portion of the slot, while the outlet dedicated to high flow rates corresponds to the wider portion of the slot. Such a device is particularly suitable for limiting over-speed by shutting only the outlet dedicated to high flow rates.
However, when supplied with fuel under icing conditions, that known device does not make it possible to prevent the output that is dedicated to low flow rates, i.e. the narrow portion of the slot, becoming partially clogged because of ice particles building up therein.